The present invention relates to a new chiral smectic C liquid crystalline polyester having excellent mechanical strengths which can be melt-molded and which is suitable for use in the field of optoelectronics because it can selectively transmit light of a specific wavelength. The present invention relates also to an optical filter comprising the chiral smectic C liquid crystalline polyester.
Liquid crystals having a specific order of the molecular orientation peculiar to them are usable in various fields, taking advantage of their molecular orientation or by controlling it. The liquid crystals, therefore, form a great industrial field. As is well known, low-molecular liquid crystals of nematic type are widely used as display elements for watches, desk-top electronic calculators or televisions and they occupy an immovable position in the display field. Recently, chiral smectic C liquid crystals having characteristic features such as high-speed operation and memory function attract public attention as liquid crystals for the display elements of the next generation which will take the place of the above-described nematic ones. Under these circumstances, the competition for the development of them is now hot. The low-molecular liquid crystals are thus used mainly in virtue of their electrooptical effects. On the contrary, high-molecular liquid crystals are used mainly as mechanical materials in virtue of their high strength, high modulus and high heat resistance owing to the high molecular orientation in their liquid crystal structure. They were already put on the market under the names of Kevlar (a wholly aromatic polyamide; a product of du Pont, U.S.A.), Xydar (a wholly aromatic polyester; a product of Dartco, U.S.A.) and Vectra (a wholly aromatic polyester; a product of Celanese, U.S.A.).
It is well known that the high-molecular liquid crystals have electrooptical effects or thermooptical effects similar to those of the low-molecular liquid crystals depending on the types of them, i.e. nematic, smectic and cholesteric types, not to mention the excellent mechanical characters. The response of the high-molecular liquid crystals to an external force such as an electric field or heat is slower than that of the low-molecular ones because of a high viscosity of them and, therefore, the high-molecular ones cannot be used for the same purpose as that of the low-molecular ones. However, the orientation structure peculiar to the types of the high-molecular liquid crystals can be fixed and, accordingly, the optical characters peculiar to the respective structures can be fixed. This is a great characteristic feature of the high-molecular liquid crystals, which the low-molecular liquid crystals do not have. Attempts have been made at preparing optical elements in virtue of this characteristic feature. As for those made of high-molecular cholesteric liquid crystals, a visible light-reflecting polymer complex comprising a cholesteric liquid crystalline polypeptide fixed with an amorphous polymer, said polypeptide having a spiral pitch capable of selectively reflecting light of a wavelength in the visible light region, is disclosed in Japanese Patent Laid-Open No. 139506/1981.
Further, a notch filter made of a visible light-reflecting polymer complex prepared as described above is disclosed in Japanese Patent Laid-Open No. 191203/1985. However, these optical elements prepared by fixing the high-molecular cholesteric liquid crystals have the fixed character of the cholesteric liquid crystals, i.e. a character of selectively reflecting light of a special wavelength from the incident light. Accordingly, from the viewpoint of the transmitted light, most of the light other than the one reflected and cut is transmitted. This is a serious problem. This fact means that this filter cannot be used as a transmitting filter through which only light of a special wavelength is selectively transmitted. This limits the practical use of the filter to a narrow range.
When the high-molecular liquid crystals are shaped into a film or a thin film to prepare elements, the molecules usually orient in parallel with the film surface. In case of the cholesteric liquid crystals, the cholesteric layer is in parallel with the film surface. Therefore, in an incident light which is at a certain angle to the face, a part thereof of a special wavelength is selectively reflected depending on the cholesteric pitch. Namely, the optical elements prepared by fixing the high-molecular cholesteric liquid crystals are essentially selective reflection filters. Further chiral smectic C liquid crystals are known as liquid crystals having the same periodicity of the molecular orientation order as that of the cholesteric liquid crystals. In the chiral smectic C liquid crystals, the orientation vector of the smectic molecular layer is slowly twisted at a certain angle and, as a whole, the orientation vector has a certain spiral structure. Let a layer distance required for the orientation vector to make one revolution be one pitch, the liquid crystals have such a periodic structure that the pitch is the base unit. Therefore, the optical properties of the liquid crystals depend on the pitch. In the films or thin films of the chiral smectic C liquid crystals, the molecules are arranged in parallel with the face but the smectic C layer is, therefore, arranged perpendicularly to the face. This is an essential difference from the structure of the cholesteric liquid crystals. The incident light on the surface of the film is reflected by the chiral smectic C layer which is perpendicular to the film surface and then travels onto the other side of the film surface. As a result, the light of a special wavelength selectively passes through the film. Thus, a transmitting filter is obtained
If a film or thin film comprising fixed high-molecular chiral smectic C crystals is produced on the basis of this principle, the film can be used as a transmitting filter which selectively transmit light of a particular wavelength. However, no polymer capable of forming a clear chiral smectic C liquid crystal phase which can be fixed has been known as yet. For example, J. C. Dubois et al. reported polyacrylic ester derivatives having a structure of the following formula in which the side chain has a mesogen containing a chiral unit [Mol. Cryst. Liq. Cryst., 137, 349 (1986)]. ##STR3##
According to X-ray diffractometry of polymers of the above formula wherein R is H and n is 2, R is CH.sub.3 and n is 11, or R is Cl and n is 11, Dubois et al. found that the polymers had a smectic layer structure and that the major axis of the mesogen of the side chain inclined to the normal. From these facts, they suggested that these polymers formed a phase which was supposed to be chiral smectic C within a certain temperature range. However, they were silent on the results of optical observation or pitch length. The formation of the chiral smectic C phase was thus not proved and no report dealing with the fixation was given. Further when the teaching of the report of Dubois et al. is to be actually practiced on an industrial scale, it has defects in that multiple steps are required for the production of the monomer having such a side chain to make the operation quite troublesome and that a high molecular weight cannot be obtained even when the monomer is subjected to the polymerization and a film formed from the polymer thus prepared from the monomer has only a poor strength. In addition, V. P. Shibaev et al. reported polyacrylic ester derivatives similar to those described above [Polymer Bulletin 12, 299 (1984)]: ##STR4## wherein m represents a number of 6 to 12 and no definition is given to R, R.sub.1 and R.sub.2.
According to X-ray diffractometry, Shibaev et al. found that these derivatives had a smectic layer structure, that the major axis of the mesogen of the side chain inclined to the normal and that spontaneous polarization was observed. From these facts, they concluded that the polymers form a chiral smectic C liquid crystal phase. However, like the polymers of Dubois et al., the polymers of Shibaev et al. have defects when the teaching of their report is to be practiced on an industrial scale. The defects are, for example, that the production of the polymers is quite difficult and that the polymers having a high molecular weight cannot be easily obtained. These chiral smectic C liquid crystalline polymers of the polyacrylic acid derivative type are produced by introducing the low-molecular chiral smectic C liquid crystal as they are into the side chain thereof and, therefore, they can be regarded to be modified low-molecular liquid crystals. If high-molecular chiral smectic C liquid crystals of the main chain type containing a chiral component in the main chain can be produced, the above-described defects can be overcome and a material which can be fusion-molded and which has a high strength and a high industrial value can be obtained. Although the production of polyesters and polyamides containing a chiral component in the main chain thereof was reported, the products are mostly cholesteric liquid crystals. Further, although some processes for producing nematic and smectic liquid crystals are known, no products having a distinct chiral smectic C phase were reported yet at all [L. L. Chapoy, "Recent Advances in Liquid Crystalline Polymers", Elsevier Applied Science Publishers, London and New York, 1985, p. 28].
The inventors have noted chiral smectic C liquid crystalline polymers as suitable starting materials for optical elements which can selectively transmit light of a specific wavelength. After intensive investigations made for the purpose of finding polymers which are free from the defects of the above-described polyacrylic ester derivatives, which can be produced easily on an industrial scale and which satisfy the requirements that they are capable of forming a distinct chiral smectic C phase, that they can be easily fixed, that they can be fusion-molded and that they have sufficient mechanical strengths, the inventors have found that polyesters containing a particular mesogen and a particular chiral component form distinct chiral smectic C liquid crystals and solve the above-described problems.